1. Field of the Invention
This invention relates to the transportation of semi-submersible drilling vessels, in particular those having fixed dynamic positioning thrusters.
2. Description of Related Art
Semi-submersible drilling vessels for conducting offshore drilling operations utilize buoyant pontoons, also known as lower hulls or floaters, which support a plurality of vertically extending columns or caissons, the upper portions of which carry a working platform. A plurality of thruster assemblies may be secured to the bottom of the pontoons for dynamically positioning the vessel at a fixed location in deep offshore waters, such as over a wellbore. Thruster assemblies are available from, for example, Wärtsilä Corporation of Helsinki, Finland and Rolls-Royce of London, England. A typical thruster assembly may cost $1.5 to $2 million, and eight (8) thruster assemblies may be attached to the pontoons of a single semi-submersible rig.
Semi-submersible rigs are typically transported in the ocean on self-propelled Heavy Transport Vessels (HTV), during what is known as a “dry tow,” since he rig is on the deck of the HTV and out of the water. While the rig is positioned on the HTV, the thrusters are typically suspended over the side of the HTV and may be exposed to the slamming and dragging forces of the waves during transport. A wave slamming force may be caused by a wave hitting the surface of the thruster and causing an impact load. A wave dragging force may be caused by dragging the thruster in the water causing a relatively steady force on the thruster.
Rig owners and/or oil and gas operators do not want to risk damage to the thrusters during transport since the damage may render the rig unusable until the thrusters are repaired. The repairs usually take a significant amount of time. Semi-submersible rigs may rent for $500,000 per day, so the loss of operational time is costly. A replacement thruster may not be available for six (6) months. Suppliers may deny warranty claims due to the excessive forces and subsequent damage experienced during transport.
One proposed solution is to remove the thrusters and reinstall them after transport. However, this solution is costly since it may take at least fourteen (14) days of critical time to remove and reinstall the thrusters, resulting in the loss of millions of dollars in wasted rig down time. In addition, the thruster supplier usually charges significant additional fees for the removal and reinstallation of the thrusters.
Another proposed solution is to tow the semi-submersible vessel in the water with one or more tug boats, known as a “wet tow.” The thrusters remain below sea level during a wet tow. However, this solution takes significantly more time than a dry tow because the semi-submersible rig has to be pulled at about one-half the speed used during a dry tow. Often the semi-submersible rig must be transported from one part of the world to another, so the loss in time and money is significant. The semi-submersible day rate is high compared to the HTV day rate. Moreover, during a wet tow, a thruster may be damaged if it impacts some obstruction below sea level.
Some semi-submersible rigs have thruster assemblies that are retractable. However, such rigs and retractable assemblies are expensive.
It would be desirable to protect the thruster assemblies during the transport of a semi-submersible vessel in a dry tow without removing the thruster assemblies.